Voice Over Internet Protocol (VOIP) networks, notably Internet Protocol (IP) Multimedia Subsystem (IMS) networks as defined by the 3rd Generation Partnership Project (3GPP), are designed around a set of functional entities, within this document also referred to as nodes, each performing particular tasks.
Most of the nodes in a VOIP network, such as an IMS network, are deployed in redundant or geographically distributed method, as is common in telecommunication networks. Reasons include: load sharing (e.g. due to the number of subscribers in the network or due to the call density), geographic distribution (e.g. because of the size of the geographical area to cover) and reliability (e.g. switching over to another node in the case of temporary node outage/link failure or in the case of node maintenance). E.g. when a Serving Call Session Control Function (S-CSCF) entity is temporarily not operational, due to failure or due to planned maintenance, subscribers may be registered in another S-CSCF which will take over the tasks of the inoperational S-CSCF. Likewise, when a P-CSCF is temporarily not operational, the subscriber would have to register with another P-CSCF.
In one known embodiment, the Home Subscriber Server HSS accepts a request for subscription data from one S-CSCF, whilst the HSS has marked the subscriber as currently being registered with another S-CSCF.
In another known embodiment, frequent re-registrations are applied. Re-registrations are needed to keep a binding between a public user identity and a contact address (or simply ‘binding’) in S-CSCF alive. When a Session Initiation Protocol User Agent (SIP-UA) performs re-registration, the registration-related data in the IMS network is (re-)synchronized at that moment. If due to network problems the subscriber's registration had to be moved to another S-CSCF, then resulting from the re-registration, the SIP-UA, P-CSCF, S-CSCF and HSS are again synchronized. By configuring the S-CSCF to instruct a SIP-UA to apply very frequent re-registrations, the time that a subscriber would be non-contactable due to node outage is minimized, at the expense of increased network signalling, which, in commercial IMS deployments, amounts to a large percentage of the total SIP signalling.
When a P-CSCF becomes inoperational, (regular) re-registration by a SIP-UA would not resolve the problem. The SIP-UA will attempt to register with the same P-CSCF as ‘discovered’ prior to initial registration.